1. Field of the Invention
The present invention relates to a disk drive motor for driving recording disks and, more particularly, to a disk drive motor which is required to operate at high speed with high stability.
2. Description of the Related Art
Various types of recording disks have been proposed and used for the purpose of recording and reproduction of data, such as compact disks (CD), floppy disks (FD), magneto-optical disks (MO), mini-disks (MD), digital video disks (DVD), hard disks (HD), and so forth. Different types of recording disks employ different recording/reproduction methods and have different specifications in regard to the size or capacity of stored data, disk driving speed, recording density, and so on, as well as disk materials and prices. Consequently, drive motors of different specifications are used for driving different types of recording disks.
Nowadays, there is a trend towards a higher degree of sophistication and a greater size of electronic data, as image data are handled more than text data. This has given rise to the demand for inexpensive recording disks and disk drives which are capable of performing quick storage and reproduction of large volumes of information.
For instance, CDs were initially used as music recording/playback media but are nowadays applied for spreading use as CD-ROMs which are major storage disks for computers, by virtue of their advantages over other types of media. This type of storage media offers greater storage capacity and shorter operation time, i.e., reduced seek time, permitting much higher speed of rotation by means of high-speed disk drive motors, thus affording disk rotation speeds 20 times as high as that of music CDs.
FIG. 1 shows the construction of a conventional disk drive motor.
The conventional disk drive motor shown in FIG. 1 has a substantially cylindrical sleeve 3 which at its lower end fits in an opening 2 formed on a base member 1 as a part of a chassis of a disk drive device. The lower opening of the sleeve 3 is closed by a tabular member 4 which carries a thrust bearing member 5. A sleeve bearing 6 made of an oil-impregnated metal or a wear-resistant resin is received in the sleeve 3.
A stator 8 includes a stator core 8a which is secured to the outer surface of the sleeve 3 and stator coils 8b wound therearound. A shaft 9 is rotatably supported by the sleeve bearing 6 such that it is in contact at its lower end, with the thrust bearing member 5 while its upper end projects beyond the upper end of the sleeve 3. A rotor hub 10 made of non-magnetic material such as aluminum is fixed to the upper end of the shaft 9. A yoke 11 made of magnetic material such as iron is fixed to the rotor hub 10.
The yoke 11 has a disk-shaped base portion and a cylindrical skirt portion which is integrally formed with the base portion and extends downward from the radially outer end of the base portion. The sleeve 3 is received in a central opening formed at the center of the base portion of the yoke 11. The inner peripheral edge of the base portion defining the central opening is fixed to a lower end portion of the rotor hub 10. An annular rotor magnet 12 is fixed to the inner peripheral surface of the cylindrical skirt portion of the yoke 11, so as to radially face the stator 8.
A turntable 14 which is secured to the outer peripheral surface of the rotor hub 10 carries a recording disk D, through an intermediary buffer member 15. A clamp magnet 16 is provided in a recess formed on the top of the rotor hub 10, such that the upper surface of the clamp magnet 16 is flush with the top surface of the rotor hub 10. The clamp magnet 16 magnetically attracts a disk pressing means (not shown) on the disk drive unit, thereby fixing the recording disk D.
In operation, electrical current supplied to the coils 8b of the stator 8 serves to generate magnetic force which acts between the stator 8 and the rotor magnet 12 so as to induce a torque for rotation. As a consequence, the rotor magnet 12, the yoke 11, the rotor hub 10 and the shaft 9 rotate as a unit, relative to the stator 8 which is stationary, whereby the turntable 14 and, hence, the recording disk D thereon rotate in a predetermined direction.
When such a conventional motor is driven at high speed, there occurs a problem which is not serious when the conventional motor is driven at low speed. Namely, high speed driving of the motor makes is difficult to control and regulate motor performance such as run-out of the rotary part, vibration and noise of the motor. In particular, when the run-out of the rotary part increases, storage and reproduction of data become less reliable.
Run-out, vibration and noise are considered to be attributable to slight local dimensional errors which produce only negligibly small effect in low speed driving but shows serious effect on high speed driving as described above, resulting in imbalance of motor parts under rotation.
It is true that rotational performance of the conventional motor structure shown in FIG. 1 can be improved to some extent when, for example, the shaft 9 and the sleeve bearing 6 are machined, finished and mounted with higher degree of precision. This solution, however, is still unsatisfactory from the viewpoint of production costs.
Rotational performance is also affected by any dimensional error of the recording disk, particularly when the rotation speed is high. When production costs of disks are considered, however, it is difficult and impractical to achieve a higher dimensional precision of recording disks.
It is materially impossible to completely eliminate any dimensional error and mass imbalance of the rotary system including the motor and the disk. Under this circumstance, a demand exists for a measure which improves the rotational performance while allowing dimensional error and mass imbalance to some extent.